Any superconductor cable must include, in addition to the superconductor, stabilizing conductor and passage for liquified gaseous coolant, an enclosing conduit or sheath which will retain the cable components. When the superconductor is an intermetallic compound such as Nb.sub.3 Sn, it is often times advantageous to form the compound in situ, that is after the essentials of the cable have been constructed, assembled and formed. This is true because intermetallic compounds are often very brittle and will rupture if subjected to more than a minimum of deformation. On the other hand, the precursors of the intermetallic compound can usually be obtained in ductile form. Generally, formation of the superconductive intermetallic compound (or composition) entails heating of the precursor materials to temperatures of the order of 1000.degree. K. or higher for a considerable time period. The formed cable containing the intermetallic superconductor is then cooled, eventually to cryogenic temperatures of about 4.degree. K. In order to prevent damage due to thermally induced strain and to minimize lowering of the critical current carrying capacity of the superconductor, the sheathing of the cable should reasonably match the superconducting compound in coefficient of expansion over the range of about 0.degree. K. to 1000.degree. K. In addition, the cable sheath or conduit must have adequate mechanical characteristics at cryogenic temperatures and adequate properties at 1000.degree. K. or higher from the stress accelerated grain boundary oxidation (SAGBO) criterion.
It is an object of the present invention to provide a superconducting cable having a sheath or conduit which is adapted to perform in a satisfactory manner both during formation of the superconductive material and during use of the cable under cryogenic conditions.